feat: 金额上限校验、订阅详情展示优化、支付商品名称区分

- 硬编码 MAX_AMOUNT=99999999.99，所有金额输入（API+前端）统一校验上限
- 管理后台订阅列表改为卡片布局，Sub2API 分组信息嵌套只读展示（平台/倍率/限额/模型）
- 用户端套餐卡片和确认页展示平台、倍率、用量限制
- 订阅订单支付商品名改为 "Sub2API 订阅 {分组名}"，余额充值保持原格式

src/app/api/subscription-plans/route.ts

@@ -24,9 +24,10 @@ export async function GET(request: NextRequest) {
     const results = await Promise.all(
       plans.map(async (plan) => {
         let groupActive = false;
+        let group: Awaited<ReturnType<typeof getGroup>> = null;
         let groupInfo: { daily_limit_usd: number | null; weekly_limit_usd: number | null; monthly_limit_usd: number | null } | null = null;
         try {
-          const group = await getGroup(plan.groupId);
+          group = await getGroup(plan.groupId);
           groupActive = group !== null && group.status === 'active';
           if (group) {
             groupInfo = {
@@ -44,6 +45,7 @@ export async function GET(request: NextRequest) {
         return {
           id: plan.id,
           groupId: plan.groupId,
+          groupName: group?.name ?? null,
           name: plan.name,
           description: plan.description,
           price: Number(plan.price),
@@ -51,6 +53,8 @@ export async function GET(request: NextRequest) {
           validityDays: plan.validityDays,
           validityUnit: plan.validityUnit,
           features: plan.features ? JSON.parse(plan.features) : [],
+          platform: group?.platform ?? null,
+          rateMultiplier: group?.rate_multiplier ?? null,
           limits: groupInfo,
         };
       }),